Acutely toxic effects of thiocyanate SCN(-) were studied in brook trout Salvelinus fontinalis and rainbow trout Salmo gairdneri exposed to concentrations of up to 518 mg/l SCN(-). These fishes were capable of accumulating thiocyanate against its concentration gradient and at relatively rapid rates dependent upon the exposure SCN(-) concentration, fish size, and anionic composition of the exposure water. Uptake of thiocyanate was inhibited by external chloride, was accompanied by a decline in the plasma Cl(-) concentration, and, therefore, presumably involved substitution of SCN(-) for Cl(-) at the HCO3(-)/Cl(-) exchange sites of the gill. Endogenous conversion of thiocyanate to cyanide was detected in exposed trout. (Copyright (c) by the American Fisheries Society 1985.)